The present invention relates to microfluidic devices and the control and metering of fluid within those devices. These devices are useful in various biological and chemical systems, particularly in systems where fluid metering is important, as well as in combination with other liquid-distribution devices.
There has been a growing interest in the manufacture and use of microfluidic systems for the acquisition of chemical and biological information. In particular, when conducted in microfluidic volumes, complicated biochemical reactions may be carried out using very small volumes of liquid. Among other benefits, microfluidic systems improve the response time of reactions, minimize sample volume, and lower reagent consumption. When volatile or hazardous materials are used or generated, performing reactions in microfluidic volumes also enhances safety and reduces disposal quantities.
Traditionally, microfluidic devices have been constructed in a planar fashion using techniques that are borrowed from the silicon fabrication industry. Representative systems are described, for example, in some early work by Manz et al. (Trends in Anal. Chem. (1990) 10(5): 144-149; Advances in Chromatography (1993) 33: 1-66). In these publications, microfluidic devices are constructed by using photolithography to define channels on silicon or glass substrates and etching techniques to remove material from the substrate to form the channels. A cover plate is bonded to the top of the device to provide closure. Miniature pumps and valves can also be constructed to be integral (e.g., within) such devices. Alternatively, separate or off-line pumping mechanisms are contemplated.
More recently, a number of methods have been developed that allow microfluidic devices to be constructed from plastic, silicone or other polymeric materials. In one such method, a negative mold is first constructed, and plastic or silicone is then poured into or over the mold. The mold can be constructed using a silicon wafer (see, e.g., Duffy et al., Analytical Chemistry (1998) 70: 4974-4984; McCormick et. al., Analytical Chemistry (1997) 69: 2626-2630), or by building a traditional injection molding cavity for plastic devices. Some molding facilities have developed techniques to construct extremely small molds. Components constructed using a LIGA technique have been developed at the Karolsruhe Nuclear Research center in Germany (see, e.g., Schomburg et al., Journal of Micromechanical Microengineering (1994) 4:186-191), and commercialized by MicroParts (Dortmund, Germany). Jenoptik (Jena, Germany) also uses LIGA and a hot-embossing technique. Imprinting methods in PMMA have also been demonstrated (see, Martynova et.al., Analytical Chemistry (1997) 69: 4783-4789) However, these techniques do not lend themselves to rapid prototyping and manufacturing flexibility. Additionally, the foregoing references teach only the preparation of planar microfluidic structures. Moreover, the tool-up costs for both of these techniques are quite high and can be cost-prohibitive.
A more recent method for constructing microfluidic devices uses a KrF laser to perform bulk laser ablation in fluorocarbons that have been compounded with carbon black to cause the fluorocarbon to be absorptive of the KrF laser (see, e.g., McNeely et aL, xe2x80x9cHydrophobic Microfluidics,xe2x80x9d SPIE Microfluidic Devices and Systems IV, Vol. 3877 (1999)). This method is reported to reduce prototyping time; however, the addition of carbon black renders the material optically impure and presents potential chemical compatibility issues. Additionally, the reference is directed only to planar structures.
Various conventional tools and combinations of tools are used when analyzing or synthesizing chemical or biological products in conventional macroscopic volumes. Such tools include, for example: metering devices, reactors, valves, heaters, coolers, mixers, splitters, diverters, cannulas, filters, condensers, incubators, separation devices, and catalyst devices. Attempts to perform chemical or biological synthesis and/or analysis in microfluidic volumes have been stifled by difficulties in making tools for analysis and/or synthesis at microfluidic scale and then integrating such tools into microfluidic devices. Another difficulty is accurately measuring stoichiometric microfluidic volumes of reagents and solvents to perform synthesis on a microfluidic scale. Additionally, difficulties in rapidly prototypic microfluidic devices are compounded by attempts to incorporate multiple analysis and/or synthesis tools for multi-step analysis and/or synthesis.
When working with fluids in conventional macroscopic volumes, fluid metering is relatively straightforward. In microfluidic volumes, however, fluid metering is considerably more difficult. Most, if not all, microfluidic systems require some interface to the conventional macrofluidic world. Using conventional macrofluidic techniques, the smallest volume of liquid that can be generated is a droplet, typically ranging in volume between approximately 1-100 microliters. At the low end of this volumetric range it is extremely difficult to consistently create droplets having a reasonably low volumetric standard deviation. Applications in which fluidic metering accuracy is important include microfluidic synthesis, wherein it would be desirable to measure stoichiometric microfluidic volumes of reagents and solvents.
A known method of obtaining small droplets is to combine fluids to be metered with surfactants before dispensing the liquid through a pipet tip. But this method is unacceptable for many applications, since adding surfactants may detrimentally compromise the purity of the fluid to be metered, and it may be very challenging to remove the surfactants and purify the fluid for further processing or use.
It is further difficult to segregate a small fluid volume from a larger bulk volume within a microfluidic device. Such segregation requires the forces of cohesion (interaction between like fluid molecules) and adhesion (interaction between fluid molecules and the surrounding conduit) to be overcome. It is believed that the general dominance of surface effects over momentum effects in microfluidic systems contributes to the challenge of performing fluid metering within such systems.
Another known method for metering small volumes of fluids is to cause the fluid to flow into a receptacle at a particular flow rate for a particular period of time and integrate the flow rate over the time to determine the volume deposited in the receptacle. For example, a fluid flowing into a receptacle at a rate of one microliter per second for one second will yield a one microliter sample in the receptacle. PCT Patent Application Number WO-01/04909 A1, entitled xe2x80x9cFluid Delivery Systems for a Microfluidic Device Using a Pressure Pulse,xe2x80x9d by Orchid Biosciences, Inc. (the xe2x80x9cOrchid Applicationxe2x80x9d), discloses a system where a plurality of branches are filled with a fluid up to a capillary break. The capillary break prevents the fluid from flowing into the test cell until the fluid pressure exceeds the pressure required to overcome the impedance of the capillary break (the xe2x80x9cbreak pressurexe2x80x9d). A pressure pulse of predetermined duration and amplitude is provided to overcome the break pressure. The duration of the pressure pulse is selected so that the desired amount of fluid flows into the test cell. In other words, the pressure pulse causes the fluid to flow into the test cell at a given rate for a given period of time to provide the desired sample volume.
This approach may not achieve the desired accuracy because of hysteresis in the system resulting from the fluid compression and variations in fluidic impedance throughout the system. Moreover, inaccuracies may be amplified in larger systems where large numbers of receptacles, many at some distance from the pressure source, are served by a complex system of fluid conduits. Also, the behavior of the system may vary depending on the materials with which the device is constructed. It is known that microfluidic flow characteristics are largely the result of surface interactions between the fluid and the microfluidic conduit or chamber. Thus, different fluids may exhibit different flow properties within the same conduit. For example, an aqueous solution under a given pressure may flow more slowly than a non-aqueous solution in a hydrophobic conduit, with the reverse being true if the conduit material were hydrophilic. As a consequence of the differing flow characteristics, a pressure pulse driving a metering operation would have to be tailored to each type of material and fluid combination used in the device. However, a wide range of materials and fluids are desirable. For example, both hydrophobic and hydrophilic solvents frequently are used in analysis and synthesis operations. Also, solvents frequently have corrosive qualities that are incompatible particular conduit materials-and many solvents are incompatible with each other. Thus, a control system for a flow-rate/time based metering system would necessarily have a different control program for each possible fluid/material combination, and the introduction of new materials or fluids would require an updated control program that may require experimentation or empirical analysis to determine flow characteristics. The complexity of such a system would make it prone to operator error. Moreover, inadvertent contamination of the fluids or materials could significantly alter the accuracy of the equipment without the error being easily detected by the operator.
Accordingly, there exists a need for metering devices and methods capable of consistently metering fluids in microfluidic volumes while minimizing the need to accommodate variations in fluid flow and material properties.
In a first aspect of the invention, a method for metering a plurality of microfluidic plugs from a larger fluidic volume comprises the steps of providing a trunk channel having a fluidic outlet and a plurality of microfluidic branch channels, each having an associated fluidic impedance region. Each branch channel of the plurality of branch channels is in fluid communication with the trunk channel. A first fluid volume is supplied to the trunk channel. Each branch channel is filled, directly from the trunk channel, to each fluidic impedance region with a portion of the first fluid volume. A second fluid is used to flush the remaining portion of the first fluid from the trunk channel through the fluidic outlet while each branch channel remains substantially filled.
In another aspect of the invention, a method for metering a microfluidic plug from a larger fluidic volume comprises the steps of providing a trunk channel having a fluidic outletand a microfluidic branch channel having an associated fluidic impedance region. The branch channel is in fluid communication with the trunk channel. A first fluid volume is supplied to the trunk channel. The branch channel is filled, directly from the trunk channel, to the fluidic impedance region with a portion of the first fluid volume. A second fluid flushes the remaining portion of the first fluid from the trunk channel through the fluidic outlet while the branch channel remains substantially filled.
In another aspect of the invention, a device for metering a microfluidic plug of fluid from a larger fluidic volume comprises a trunk channel having a fluidic inlet and a fluidic outlet and a microfluidic branch channel in direct, independent fluid communication with the trunk channel. The branch channel has a fluidic impedance region. The trunk channel, branch channel, fluidic inlet, fluidic outlet, and fluidic impedance are arranged to permit a first fluid to be supplied through the trunk channel to fill the branch channel to the fluidic impedance region, and thereafter to permit the fluidic contents of the trunk channel to be flushed through the fluidic outlet while the branch channel remains substantially filled.
In another separate aspect of the invention, any of the foregoing separate aspects may be combined for additional advantage. These and other aspects and advantages of the invention will be apparent to the skilled artisan upon review of the following description, drawings and claims.